I. Field of the Invention
The present invention relates to oscillator circuits, and more particularly relates to oscillator circuits that tune over a range of frequencies with an increasing tuning gain factor as a function of increasing frequency.
II. Description of the Related Art
A phase lock loop requires an oscillator wherein the frequency of oscillation of the oscillator is controllable. A typical circuit for performing such a function is comprised of an amplifier and a resonant tank. The resulting circuit has a frequency where the gain is greater than unity and the phase is equal to zero. This frequency is the frequency of oscillation as set by the resonant tank. The relationship is most easily seen on a Bode diagram. FIG. 1 illustrates a Bode diagram for a typical oscillator. Curve 10 is representative of the gain in decibels of the oscillator as referenced to the left vertical axis and Curve 20 is representative of the phase in degrees as referenced to the right vertical axis. As indicated by Point 30, the oscillation occurs when the oscillator gain in approximately 14 dB and the phase is zero producing an oscillation at approximately 124 MHz. In general the resonant tank is comprised of at least one variable component wherein the reactance of the variable component is a function of a control signal, typically a voltage level, so that the frequency of zero phase, and consequently the frequency of oscillation, is also variable.
FIG. 2 illustrates in graphical form a standard frequency versus control signal graph. Curve 50, as referenced to the left vertical axis, represents the frequency response of the oscillator circuit in MegaHertz (MHz) to the control signal in Volts (V). A portion of the Curve 50, corresponding a control signal level between 4 and 10 Volts, is approximately linear. Line 60 is shown for reference purposes and represents a linear approximation of the control voltage from 4 to 10 Volts. If Curve 50 is mathematically differentiated with respect to the control signal, the result is the oscillator sensitivity or gain factor, K.sub.V, typically having the units Hz/Volt. Curve 70 represents K.sub.V for the oscillator of FIG. 2 as referenced to the right vertical axis. Curve 70 is typical of standard oscillators in that it is a decreasing function of increasing control voltage and frequency.
When the oscillator circuit described above is included in a phaselock loop circuit, oscillator gain factor, K.sub.V, is one of the factors which determines the behavior of the loop. FIG. 3 represents a block diagram of a typical phaselock loop. Voltage controlled oscillator (VCO)120 is an oscillator circuit wherein the frequency of the output signal increases with increasing applied voltage with a gain factor of K.sub.V. When the VCO gain is transferred into the s-domain, it becomes K.sub.V /S with the s in the denominator indicating that the VCO acts as an integrator of phase error. The output of VCO 120 is divided down to a lower frequency by divider 130 defined by: ##EQU1## where: F.sub.DIV is the frequency of the output of divider 130;
F.sub.VCO is the frequency of the output of VCO 120; and PA1 N is the integer division ratio provided by divider 130.
The output of divider 130 with phase .phi..sub.O is coupled to phase/frequency detector (PD) 100. PD 100 compares .phi..sub.O to the phase of a reference input, .phi..sub.I, and produces an output error signal proportional to the phase difference of its two inputs. The gain of PD 100 is K.sub.D having the units Volts/radian. Loop filter 110 filters the output of PD 100 and has the s-domain transfer function F(s). The output of loop filter 110 is applied to the control terminal of VCO 120.
The divider of a typical phase lock is programmable and provides the frequency selection of the output. To illustrate the frequency control of the loop, assume that the loop is closed meaning that the frequency of the output, F.sub.VCO, is exactly N times the reference input frequency (F.sub.REF). Consider the reaction of the loop if the value of N is decreased. Initially the frequency from divider 130 increases and the two inputs to PD 100 are no longer in phase. The error signal from PD 100 reflects the magnitude of the phase difference. The output of loop filter 110 decreases causing the output frequency of VCO 120 to also decrease. This mechanism continues until the two inputs of PD 100 are again in phase. The result is that the loop is "locked" at a lower frequency.
The manner in which the loop becomes and stays locked is set by the open-loop transfer function, G(s), of the phaselock loop circuit and is mathematically is defined by: ##EQU2## where all the variables are as define above. The closed-loop transfer function follows directly from the open-loop transfer function and is defined by: ##EQU3## The open-loop and closed-loop transfer functions determine many important characteristics of the loop including stability, damping factor, settling time, and phase noise.
The problem of such a phase-lock loop is both N and K.sub.V are functions of the loop output frequency. N increases in value to directly increase the phaselock loop output and K.sub.V decreases with increasing VCO frequency when using a standard oscillation circuit with the characteristics previously described. The open-loop transfer function, being proportional to K.sub.V /N, is consequently also a decreasing function of output frequency directly rendering the loop characteristics dependent on the output frequency.
The present invention discloses a dissimilar varactor diode principle voltage controlled oscillator (DDPO) wherein K.sub.V is an increasing function of oscillation frequency. In a circuit where the required set of output frequencies covers a significant range and the loop characteristics are critical, the effects of decreasing open-loop and closed-loop transfer functions could render some more difficult designs unable to be actualized with standard oscillators. However by employing a DDPO, these more difficult designs are realizable.
It is therefore the objective of the present invention to provide a novel and improved apparatus and method for implementing a voltage controlled oscillator wherein the gain factor of the oscillator, K.sub.V, is an increasing function of increasing frequency.